This invention relates to ceramic materials and more particularly to aluminosilicate ceramic materials.
Ceramics based on monoclinic BaO.Al.sub.2 O.sub.3.2SiO.sub.2 (celsian, BAS), monoclinic SrO.Al.sub.2 O.sub.3.2SiO.sub.2 (SAS), or monoclinic solid solutions of BAS and SAS are attracting considerable interest for a variety of applications due to their unique combination of high refractoriness, low thermal expansion, low dielectric constant and loss tangent both stable over a broad range of temperatures and frequencies. Dielectric ceramics, electronic packaging, and structural ceramics are all possible applications for BAS, SAS, or mixture thereof.
Celsian exists in two main crystalline modifications: monoclinic, stable up to 1590.degree. C. and hexagonal, stable from 1590.degree. C. to the melting temperature (1760.degree. C.). Even though the hexagonal modification (hexacelsian) is the high temperature modification, it tends to be the first product of synthesis (solid- and gaseous-state reactions, melt crystallization, sol-gel process, and oxidation of metals) and persists metastably throughout the whole temperature range. Hexacelsian reversibly transforms at 300.degree. C. into the low temperature orthorhombic modification. This transformation is accompanied by a significant volume change making hexacelsian unsuitable for high-temperature thermal cycling applications. Transformation of hexagonal celsian into the desirable monoclinic form is promoted by prolonged high-temperature heating, hydrothermal treatment, and by the presence of impurities or the addition of certain additives (such as B.sub.2 O.sub.3, LiF, Cr.sub.2 O.sub.3, ZrSiO.sub.3). The additives or impurities can adversely affect all properties of the ceramics, particularly their dielectric behavior and high-temperature mechanical properties.
Strontium aluminosilicate (SAS) with a melting point of 1710.degree. C. exhibits similar polymorphism. Formation of SAS by solid phase reaction is also characterized by the primary appearance of a metastable hexagonal form. However, contrary to hexagonal BAS, the hexagonal phase of SAS is very unstable and the preparation of monoclinic SAS does not pose any problem. A minimum process temperature of 1550.degree. C. is needed to produce these ceramic materials. It would be desirable for economic reasons to reduce the minimum ceramic process temperature. The monoclinic BAS, SAS, or BAS+SAS solid solution ceramic material firing temperature might be reduced by using sintering aids. However, conventional sintering aids can destroy the mechanical and dielectric properties of the final ceramic material.
It would also be desirable to increase the flexural strength and toughness of the monoclinic BAS, SAS, and BAS+SAS solid solution ceramic materials. This can be accomplished by adding reinforcing material. Silicon nitride in the form of whiskers or powder (loadings of 20-40%) greatly increases the strength and fracture toughness of celsian. Silicon nitride also has good dielectric properties (low dielectric constant and loss tangent) which are very important for some applications. Unfortunately, expensive hot pressing or hot isostatic pressing (HIP) is required to produce densified silicon nitride reinforced celsian composites.